This invention relates to electric motors, and particularly to series wound universal motors, and to tools incorporating such motors. In the known art of electric motors the problem of furnishing adequate braking of the motor upon its turning off is known. Continuing rotation by inertia is in fact a source of risk in various situations. In particular, it is desirable that portable electric tools, such as for example circular saws, grinders, etc., can be braked in a short time to allow the user to lay them down without excessive care soon after switching-off. In tools having universal motors supplied with alternating current, a braking system very widely used because of its economy is that of connecting in parallel the armature and field winding upon turning-off the tool. In this manner the counterelectromotive force generated produces a braking action on the rotor. Said system can however fail if the tool is turned off opposite a zero position of the supply current sinusoid. In this case the residual magnetic flux in the windings is nil and hence no braking force is generated. To obviate this shortcoming, various circuits have been proposed. For example, on switching-off, it has been proposed to power the motor with a direct current voltage for a brief fraction of time immediately before connection in parallel of the armature and field windings. This ensures the existence of a residual flux in the next parallel-connection phase. A circuit providing this feature is in DE-A-3820629.
Even after this elimination of the possibility of brake failure, there is another problem with both the braking techniques described above, and this is that the braking action can initially be very sharp, but then it decreases with the slowing of motor rotation. Thus the mechanical parts of the motor and, indeed, the tool in which it is fitted, are greatly stressed at the beginning of the braking process and this results in the possibility of mechanical failures. In addition, with manual tools the `jerk` of the sudden braking can cause the tool to escape from the user's hand if not firmly gripped.